Minor annoyances with properties

[eb303]

Hello all,

I've been using Python properties quite a lot lately and I've found a
few things that are a bit annoying about them in some cases. I
wondered if I missed something or if anybody else has this kind of
problems too, and if there are better solutions than the ones I'm
using ATM.

The first annoyance is when I want to specialize a property in a
subclass. This happens quite often actually, and it is even sometimes
the reason why a plain attribute is turned into a property: a subclass
needs to do more things than the superclass when the property is
updated for example. So, of course, my first try was:

class A(object):
  def __init__(self):
    self._p = None
  def _get_p(self):
    return self._p
  def _set_p(self, p):
    self._p = p
  p = property(_get_p, _set_p)
class B(A):
  def _set_p(self, p):
    ## Additional things here…
    super(B, self)._set_p(p)

And of course, it doesn't work: the property has been bound to
A._set_p in A, so any new definition of _set_p in any subclass does
not replace the set method for the property. So I always have to add a
line:
p = property(A._get_p, _set_p)
in the subclass too. This is a bit awkward to me, since I have to
specify the superclass's name (super(…) can't be used, since it should
take B as an argument, and B isn't defined yet…). Do I miss something?
Is this the way to do it, or is there a better one?


The second annoyance is when I have a property that is a list of
something. I often have to do something when the contents of the list
is modified. So basically, I often end up doing the following:
def C(object):
  def __init__(self):
    self._l = []
  def _get_l(self):
    return list(self._l)
  def _set_l(self, l):
    self._l = list(l)
  l = property(_get_l, _set_l)
But then, I have to do:
o = C()
l = o.l
l.append(42)
o.l = l
instead of just doing:
o.l.append(42)
which would seem much more natural IMHO.

Is there any not too complicated way to have o.l.append(…) call
something in C? And the same for o.l.remove(…), o.l[i] = …, and
everything else updating the list contents?

Thanks!
 - Eric -